Yes, they are existing .xlsx files, they have been modified/updated locally and therefore should be updated i.e. copied across to the remote.
They got there via an earlier rclone run. Can't say when that was as I did not check my logs every time just do a spot check every few weeks (aside: I have a backup NAS and that is backed up to two portable disks one of which is kept off site plus I have timemachine on the Mac - so I am adding this as an extra layer of protection - hence the lax checking of logs).
OneNote is a mystery. I have don't have any one note files that I know off. I do have OneNote on the Mac but it stores all its files in the cloud and can't be set to put them on my local disk or any remote disk.
So the "nameAlreadyExists" part is understandable - it does, it is/should be updated. The upload session already exists is not clear to me as I don't know what an upload session is (is this an rclone error or OneDrive error) but I guess that it thinks it is already trying to upload it somehow and the OneNote is a complete mystery but could be some helpful suggestion (applicable or not) that is added to the error message.
One final point. I did do strings $(which rclone) and it did not show the error message in rclone. No conclusive as it could be elsewhere.